[GRASS-user] meaning of "geometry"

[Dave Roberts]

Dear Markus and Anna,

I'd have to dig a little for man pages, but one interesting example is a 
series of GRASS-user postings on "grass vector model, cats and layers 
concept" from 6/13 which you were central to Markus.  At the time, the 
question was "why does adding a new layer to a map require creating a 
new map?"  And your answer (over two postings) was "Because you need to 
modify vector geometries in order to add a new layer. Categories and 
layers are first and foremost stored together with the geometries... 
[Adding a new layer] ... changes the geometry directly."

Nikos Alexandris (and I) were baffled by this because it doesn't change 
any vertices, nodes, or centroids.

Now, having studied the structure of the coor file I see that adding a 
new layer to a vector object increases the length of that record in the 
coor file, so that all records downstream of that record also have to be 
re-written.  It's simpler and safer to write a new vector object than to 
modify the original.  It would be possible to simulate the expected 
behavior by writing the new map to "tmp", deleting the original map, and 
renaming the tmp map to the original name, but obviously the programmers 
elected not to do that.  It could, however, be easily implemented in a 
shell or python script.

So now when I see "geometry" in a manual page or GRASS-user posting I 
interpret that to mean "vertices, nodes, centroids, and associated 
categories ordered by the (not necessarily consecutive) layer numbers" 
as that's the essential nature of a record in the coor file that manages 
all this.

Obviously that's more operational than conceptual so I was wondering 
where the concept of geometry is actually defined.

Thanks, Dave


On 10/24/16 14:39, Markus Metz wrote:
> On Mon, Oct 24, 2016 at 4:07 PM, Anna Petrášová <kratochanna at gmail.com> wrote:
>> On Mon, Oct 24, 2016 at 9:04 AM, Dave Roberts <droberts at montana.edu> wrote:
>>> Frequently in GRASS help files or emails in this list the term "geometry" is
>>> used.  Is geometry synonymous with a record in the coor file, or does it
>>> have a broader meaning?
>>
>> I would say geometry of features, such as points, lines, boundaries,
>> areas.
>
> and centroids. In most cases, the vector features of interest are only
> points, lines, and areas.
>
>> Depends on the particular context I guess, could be related to
>> topology too.
>
> An example of the context, i.e. a particular manual, would be helpful
> for clarification.
>
> Markus M
>
>> I am actually not sure what is in the coor file, but you
>> are not supposed to access it directly anyway.
>>
>> Anna
>>
>>>
>>> Thanks, Dave
>>> --


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